1. Field
Exemplary embodiments of the present invention relate to a method of fabricating a gallium nitride (GaN)-based semiconductor device, and more particularly, to a method of fabricating a GaN-based light emitting diode (LED) with a vertical structure by using a GaN substrate as a growth substrate.
2. Discussion of the Background
In general, because nitrides of Group-III elements, such as gallium nitride (GaN) and aluminum nitride (AlN), have excellent thermal stability and a direct-transition-type energy band structure, the nitrides of Group-III elements have recently come into the spotlight as materials for light emitting devices in visible and ultraviolet regions. Particularly, blue and green light emitting devices using indium gallium nitride (InGaN) have been employed in various applications, such as large-sized full-color flat panel displays, traffic lights, indoor illumination, high-density light sources, high-resolution output systems, optical communications, and the like.
Because it is difficult to fabricate a homogeneous substrate on which a semiconductor layer of a nitride of Group-III elements can be grown, the semiconductor layer of a nitride of the Group III elements has been grown on a heterogeneous substrate having a similar crystal structure through a process such as metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). A sapphire substrate having a hexagonal crystal system structure is frequently used as the heterogeneous substrate. Recently, there has been developed a technique for fabricating a high-efficiency light emitting diode (LED) with a vertical structure by growing epitaxial layers, such as nitride semiconductor layers, on a heterogeneous substrate, such as a sapphire substrate, bonding a support substrate to the epitaxial layers, and then separating the heterogeneous substrate using a laser lift-off technique or the like. Because the heterogeneous substrate and the epitaxial layer grown thereon, have different physical properties, the growth substrate can be easily separated at an interface therebetween. The separated heterogeneous substrate may be reused as a growth substrate or for other uses through surface treatment.
However, the epitaxial layer grown on the heterogeneous substrate has a relatively high dislocation density as a result of lattice mismatch and a difference in thermal expansion coefficients of the epitaxial layer and the growth substrate. It is known that the epitaxial layer grown on the sapphire substrate generally has a dislocation density of 1×108/cm2. There is a limitation on improvement of the light emitting efficiency of the LED using such an epitaxial layer having the high dislocation density.
In order to reduce a crystal defect resulting from growth of the epitaxial layer on the heterogeneous substrate, there has been a recent attempt to use a GaN substrate as a growth substrate. The GaN substrate is a homogeneous substrate in connection with a GaN-based semiconductor layer grown thereon, and thus a high-quality GaN-based semiconductor layer can be grown by substantially reducing the number of crystal defects.
However, because the GaN substrate is a homogeneous substrate in connection with the GaN-based semiconductor layer grown thereon, it is difficult to separate the growth substrate from the GaN-based semiconductor layer at an interface between the substrate and the epitaxial layer. Particularly, because a laser used to separate a conventional sapphire substrate is entirely absorbed into the GaN substrate, the laser cannot be applied to the separate the GaN substrate.
Accordingly, studies regarding a technique for separating a GaN substrate using a laser have not yet been conducted, and it is presently acceptable to remove the GaN substrate by means of polishing. In this case, the GaN substrate cannot, however, be reused, resulting in increased total production costs.